Bensham (from Bathstore) shower bath mixer problem

Hello
I bought a Bensham Bath/Shower Mixer from Bathstore about 2 years ago. Sometimes we have to turn the shower on and off several times and twist the
temperature knob back and forth several times in order to get the hot water to come through. We don't have any issues with the hot water anywhere else. It's as if something is loose or not connected properly. It was ok for
the first year and a half we had it, but it is getting worse everytime now. Any
idea what the problem could be please ? The hot water does eventually come through if we repeat the process often enough.
Thanks
Dave